We just had this problem with our vf2 X axis. There are only 2 probable fault causes here, one simple and cost nothing and the other will set you back $1500.
First start by disconnecting the two cables that power the servo. Clean the connectors with denatured alcohol and blow them out with air. Over the years stuff gets into the connector causing the encoder to loose counts. This is a common problem with the X axis since it is inside the machine.
If that does not solve the problem, it is likely the servo. Unfortunately you cannot replace just the encoder on the brushless drives so Hass will stick you with a $3000 bill for the unit, but if you return the core you get $1500 back.